1. Field of the Invention
The present invention is directed to a method of and an apparatus for laminating covering material in sheet form to a foamed plastic substrate and, more particularly, it is directed to the treatment of the surface of the plastic substrate prior to the laminating operation.
2. Description of the Prior Art
The utility of a laminated sheet consisting of a foamed thermoplastic core sandwiched between two sheets of covering material has long been recognized. In U.S. Pat. No. 2,770,406 a carton is disclosed constructed from a laminate of a polystyrene foam core sandwiched between two sheets of kraft paper. Such a laminate can be used to replace corrugated paper board or wood in many carton and crate applications, particularly where insulation is required for frozen items, such as ice cream, frozen meat and vegetables, or as a shipping carton for chilled food and produce. In such known applications, the density of the polystyrene foam can vary in the range from 2 to 10 pounds per cubic foot with the commonly preferred density being about 3 pounds per cubic foot. If the density of the foam approaches 10 pounds per cubic foot, it involves an unnecessary expense for the plastic material, however, if the foam is expanded to an extent that the gas bubbles formed become too large, the resultant product when cooled will tend to be too brittle and crack upon the application of any pressure. Accordingly, foamed material having a density of about 3 pounds per cubic foot appears to be most suitable for lamination between sheets of kraft paper. The above patent suggests a convenient method of forming the laminate by placing a sheet of polystyrene containing a foaming agent between two sheets of kraft paper so that heating and foaming takes place in the polystyrene while pressure is applied against the heated kraft paper sheet, whereby the parts of the laminate will adhere together without using any adhesive material.
In U.S. Pat. No. 3,062,698 another laminate of polystyrene foam and kraft paper is described where heat is applied to the foamed polystyrene layer through the kraft paper so that the polystyrene is neither heated nor softened before it is contacted by the paper.
In another process for forming laminate disclosed in U.S. Pat. No. 3,823,047, the kraft paper is supplied to the laminating operation at a temperature above the melting point of the foamed material while the foamed material is introduced at ambient temperature.
In the past difficulties have been experienced in achieving adequate adherence of the covering sheet to the foamed material without damaging the foamed material. Unnecessarily heating the foamed material, or applying more than a light pressure in effecting the lamination, tends to have a deleterious effect on the laminated product.